Conversion of natural gas to more valuable clean fuels, such as gasoline and diesel, has been in the center of attention for the previous fifty years by making use of the Fischer-Tropsch (F-T) process. Since in recent years new huge natural gas reserves have been discovered and explored worldwide (about 175,000 billion standard cubic meter at the end of 2003), there is a strong interest for developing means for exploitation and commercialization of this valuable resource of energy.
One of the most attractive processes for monetization of natural gas is the so called GTL (Gas to Liquid) process, including the production of synthesis gas, F-T synthesis and finally upgrading to produce clean fuels and valuable solvents.
Several reactor configurations have been developed to produce liquid products by Fischer-Tropsch synthesis such as Fixed-Bed (ARGE), Circulating Fluidized-Bed (Synthol) or Entrained-Bed, Fixed Fluidized-bed (advanced synthol) and Slurry Bubble Column (SBC) Reactors.
Gas—Solid reactors such as Fixed bed and Fluidized bed reactors encounter several problems, such as disability to remove the large amounts of released reaction heat, low conversion efficiency, non-isothermal reaction, local overheating of catalyst and low lifetime period of catalyst.
An overview on the history and some characterization of these reactors is presented in U.S. Pat. Nos. 4,670,472 and 6,217,830. Due to problems appearing with Gas-Solid reactors, other kind of the reactors, namely slurry- or three-phase reactors, were developed.
SBC reactors can handle large amounts of reaction heat in such a manner that the reaction temperature can be controlled easily, the reaction is carried out isothermally and a local overheating of the catalyst (hot spots) and then the catalyst deactivation are prevented.
In spite of the before-mentioned advantages of the SBC reactors, their application depends on the probability of utilization of a reliable technique for separation of the catalyst from the wax product. In other words, an SBC reactor can be used in a commercial scale provided that a cost-effective technique for the catalyst/wax separation is available.
During the last 50 years a lot of efforts have been made to propose methods for catalyst/wax separation. Several techniques, such as internal and external filtration, natural or forced sedimentation, magnetic separation, vacuum distillation, and chemical conversion have been used. Internal and external separation, using different filter elements, such as woven mesh, sintered metal etc has been described in various publications. Natural and forced sedimentation have, for example, been presented in WO 98/27181. Application of magnetic filtration is described in U.S. Pat. No. 4,605,678.
Internal or external filtration is one of the oldest techniques in the field of catalyst/wax separation. Filtration, in the form of pressure filtration, either performed inside the reactor or outside it, cannot keep the liquid level of the reactor at a desired value, due to high viscosity of the slurry (4-8 cp at 200° C.). In addition, internal filtration always has the plugging risk, which may lead to premature shutdowns of the reactor. Furthermore, the low separation rate which is caused by the high viscosity of the wax, and also being obliged to use catalysts of a relatively large particles size range (about 30-80 microns) to improve the filtration efficiency, results in the low efficiency of the reactor and its low conversion.
Natural or forced sedimentation techniques such as settlers and centrifuges can not be regarded as reliable techniques in catalyst/wax separation due to their inability in efficient sedimentation of the catalyst in a short time and also inability to gain favorable particle concentrations (1-2 ppm). In addition, these techniques are often performed in batch or intermittent mode, and only can be used as a preliminary separation step in catalyst/wax separation due to high viscosity of the wax and the low rate of the sedimentation. In this field, other techniques, such as addition of agglomeration agents and surface tension reduction agents to improve settling time, have not found wide application because of the mentioned problems of the settling techniques and difficulty of separation of the added reagents.
Results of U.S. Pat. Nos. 4,605,678 and 5,827,903 disclosing magnetic separation technique for catalyst/wax separation show that the technique results about 100-900 ppm catalyst loss. So, the magnetic separation is not reliable for catalyst/wax separation. Other techniques in this field such as High Gradient Magnetic Separation technique (HGMS) are relatively efficient, but they are very expensive. Because of high costs for the super conductor and also the high annual costs for electricity for HGMS filters, this method does not have a chance to be used in commercial scales.
Some other separation techniques, such as chemical conversion, have just been used in the case of Fe catalysts, but have lead to unacceptable results. Vacuum distillation can not be regarded as a reliable separation technique because of the remaining of more than 80% of the wax as a heavy residue in the vacuum distillation tower, and also because of the tendency of wax hydrocarbons for thermal cracking during distillation.
Due to the advantages of the SBC reactors, in recent years Fischer-Tropsch synthesis in the slurry phase has been focused by the research and technology centers. Almost all of them use internal or external filtration or combined internal and external filtration. But they use catalysts with a large particle size (30-80 microns) that leads to lower reactor efficiency and large catalyst make-up (50-200 ppm/feed), not desired as to economic aspects. In addition, due to high wax viscosity, they use filters with high pore size to prevent the plugging problem of filter and premature shutdowns, and hence unnecessary loss of the catalyst cannot be avoided.
Supercritical Fluid Extraction (SFE) is one of the most modern techniques presented in the field of separation. This technique has a wide range of applications in liquid-liquid and liquid-solid extractions during the last years of the twentieth century. In recent years, this technique has been used for the separation of components of medicines, herbal essences and odorants in the chemical, pharmaceutical and food industries.
U.S. Pat. No. 4,162,965 describes the principles of application of a solvent for solid particles separation from hydrocarbon mixtures. The document discloses steps of separation of solid particles from oily hydrocarbons through their contact with hydrocarbon solvents.
U.S. Pat. No. 4,559,133 describes the application of supercritical fluids in the separation of solid particles from oily hydrocarbons. Mixing steps of the supercritical solvent with oily hydrocarbons and the consecutive separation of solid particles and recovering of the solvent are described in this document.
U.S. Pat. Nos. 6,114,399 and 6,217,830 describe the application of supercritical fluid extraction in catalyst/wax separation in the product of slurry reactors.
However, the methods according to the state of the art are not very efficient and fail to provide an effective and continuous catalyst/wax separation.